public class 空心正方形 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入正方形边长：");
        int a = sc.nextInt();
        for (int i = 1; i <= a; i++) {
            for (int j = 1; j <= a; j++) {
                if (i == 1 || i == a || j == 1 || j == a) {
                    System.out.print("* ");
                } else {
                    System.out.print("  ");
                }
            }
            System.out.println();
        }
    }

}
